Guava

Multisets

• Type of collection

Collection behaviour

• Can it have duplicates?
• Is ordering significant? (for equals())
• Iteration order
  • insertion-ordered? (Linked list) comparator-ordered? (Tree) user-ordered? (Array)
  • something else well-defined?
  • or it just doesn't matter?

In general, the first two determine the interface type, and the third tends to influce your choice of implementation

List vs. Set

• Set: unordered equality, no dups
• List: ordered equauality, can have dups

Diagram (31:50)

                 Ordered?
             Y             N
Dups? +--------------+----------+
  Y   |     List     | Multiset |
      +--------------+----------+
  N   | (UniqueList) |    Set   |
      +--------------+----------+

Multiset: unordered equality, can have dups

• == Bag
• use cases,
  • hand of cards, compare same hands
  • are these Lists equal, ignoring order?
  • histograms, what distinct tags am I using on my blog, and how many times do I use each one?

Example code, before (36:07)

Map<String, Integer> tags
  = new HashMap<String, Integer>();
for (BlogPost post : getAllBlogPosts()) {
  for (String tag: post.getTags()) {
    int value = tags.containsKey(tag) ? tags.get(tag) : 0;
    tags.put(tag, value + 1);
  }
}

• distinct tags: tags.keySet()
• count for "java" tag: tags.containsKey("java") ? tags.get("java") : 0;
• total count: // oh crap...
• Java tutorial shows proper way of doing this!!! (have a look)

Example code, after(38:27)

Multiset<String> tags = HashMultiset.create();
for (BlogPost post : getAllBlogPosts()) {
  tags.addAll(post.getTags())
}

• distinct tags: tags.elementSet();
• count for "java" tag: tags.count("java")
• total count: tags.size()

Example, after after (40:10)

• if need to remove/decrement? (Multiset supports it)
• concurrency? (instead of locking the entire map, use ConcurrentMultiset)

Part 2 (http://www.youtube.com/watch?v=9ni_KEkHfto)

Multiset API (00:41)

• Everything from collection plus
• count, add, remove, setCount, etc